The invention relates to a transduction element and more particularly to a transduction element of a semiconductor material. The invention further relates to methods for the manufacture of the transduction element and to a pressure transducer incorporating the transduction element.
The development and use of piezoresistive pressure transducers based upon the use of a semiconductor material with for example diffused-in resistances has become of increasing significance in recent years, especially in the field of automation. The measuring element of such pressure transducers is a plate of preferably, monocrystal silicon arranged on an insulating substrate, such as glass, with an elastic diaphragm region provided on the crystal and which undergoes a bending movement under the effect of the pressure being measured. The resistances are secured to or diffused into the diaphragm region at the positions with the highest radial and tangential stresses, with the resistances being connected in a known manner to full or half measuring bridge networks.
Silicon as a monocrystal material has special mechanical properties in that it exhibits practically no creep or aging effects. Silicon is therefore an ideal semiconductor material for the formation of diaphragm plates, especially as it can be machined mechanically or chemically with high precision and has especially good electrical properties. The essential disadvantage of silicon is that it has a lower coefficient of thermal expansion by a factor of 4 to 6 than the steels which are conventionally used for the pressure transducer housing, especially the austenitic stainless steels. Therefore, temperature changes can easily lead to stresses and distortion of the silicon diaphragm plates secured to such steels, and this can lead for example to changes in the zero datum point.
The development work over the last few years has therefore concerned itself with these highly undesirable stress problems, and various measures have been proposed in order to at least partially avoid these disadvantages of piezoresistive pressure transducers based upon the use of silicon diaphragm plates. Thus, in EP-Al-0033749 it is proposed to secure the silicon diaphragm plate by means of a plastic or silicon rubber material in an elastically extensible and shearing manner to the metallic assembly plate of the sensor housing, so that the expansion differences between silicon and metal are absorbed elastically. However, this measure always results in reaction forces being exerted on the silicon diaphragm plate, so that it is only suitable for pressure sensors which have low demand requirements in terms of stability and constancy of the zero point. Moreover, plastic materials and especially silicon rubber exhibit a very high coefficient of thermal expansion and a pronounced hysteresis in relation to the stress/expansion ratio. Both these properties can cause additional errors in the pressure measurement.
From West German Patent Specification DE-Al-29 38 240 known there is known a piezoresistive pressure transducer based upon the use of a transduction element in which the silicon diaphragm plate is secured to an insulating glass plate by an anodic bonding, for which heating to the temperature range of 360.degree. C. to 500.degree. C. is necessary. Since glass has a higher coefficient of thermal expansion than silicon, the laminate of the two plates produces internal stresses, and consequently the silicon diaphragm plate is put laterally in the state of pressure stress. In extreme cases this can lead to the diaphragm going from a stable position to a "folded over" position (tin lid effect). Such an instability, even if the extreme case is not actually reached, changes the characteristics or calibration curve in the most undesirable way and is responsible for various deviations from linearity. The zero point in particular is strongly dependent on the temperature.
Finally, it is known from West German Patent Specification DE-Al-30 09 163 to provide a piezoresistive pressure transducer based upon the use of a silicon diaphragm plate which is secured by means of a thick soft solder layer to an assembly plate made of an iron-nickel alloy. Soft solder has a considerably higher coefficient of thermal expansion than silicon and therefore in the event of temperature changes likewise results in bending of the diaphragm plate. Such pressure transducers are therefore not suitable for precision measurements.